It has been known that properties such as heat insulation, sound absorption, buoyancy, elasticity and weight reduction are improved by foaming plastics either mechanically by using a reactive gas or by using foaming agents. Such foamed plastics are widely used as soundproofing materials, heat insulation, building materials, lightweight structural materials, packaging materials, insulating materials, cushioning materials, and quakeproof materials, as well as in footwear, automobile interiors, and in other products. Many usable resins are known, and various foaming techniques are also known.
To save energy, resources and space, it is desirable to improve the heat insulation and soundproofing capabilities in materials that are as thin as possible.
To improve the heat insulation or soundproofing performance per unit thickness, it is desirable to maintain a vacuum inside the foamed plastics. Vacuum pumps, as generally known, are incapable of maintaining individual vacuums in the foamed plastics, and consequently the heat insulation and soundproofing capabilities are not satisfactory.